(a) Field of the Invention
The present invention relates to controlling circuits for electric shutters and more particularly to improvements in an electric circuit wherein a shutter blade opening motion and closing motion are made to be started respectively by a pair of electromagnets to be energized by discharge currents of capacitors.
(B) Description of the Prior Art
A conventional example of this kind of controlling circuit is shown in FIG. 1 in which symbol E.sub.0 indicates a current source battery, S.sub.0 indicates a current source switch, L.sub.1 indicates a driving coil serving to start a shutter blade opening motion, L.sub.2 indicates a driving coil serving to start a shutter blade closing motion, C.sub.1 and C.sub.2 indicate capacitors for respectively energizing the driving coils L.sub.1 and L.sub.2, R.sub.1 and R.sub.2 indicate resistors for respectively charging the capacitors C.sub.1 and C.sub.2, D.sub.1 and D.sub.2 indicate diodes for respectively preventing reverse flows, T.sub.1 and T.sub.2 indicate transistors to be respectively used as switching elements for connecting the coils L.sub.1 and L.sub.2 respectively to the capacitors C.sub.1 and C.sub.2, Rv and C.sub.3 indicate respectively a variable resistor and a capacitor forming a CR delay circuit for controlling the exposure time and S.sub.1 indicates a switch for starting the operation.
This circuit operates as follows. First of all, when the current source switch S.sub.0 is closed, the capacitors C.sub.1 and C.sub.2 will be charged by the current source battery E.sub.0 respectively through the diode D.sub.1 and resistor R.sub.1 and through the diode D.sub.2 and resistor R.sub.2. Then, when the switch S.sub.1 is closed, the transistor T.sub.1 will be on, a large electric current will be instantaneously fed to the driving coil L.sub.1 from the capacitor C.sub.1, a locking lever not illustrated will be moved by the energization of the driving coil L.sub.1 and a shutter blade opening motion will be started. In such case, the current from the capacitor C.sub.1 will be checked by the diode D.sub.1 and therefore will not flow into the driving coil L.sub.2. On the other hand, by the closing of the switch S.sub.1, the transistor T.sub.3 will be on, the transistor T.sub.4 will be off and therefore charging the capacitor C.sub.3 through the variable resistor Rv will be started. When a certain time, that is, a proper exposure time elapses, both transistors T.sub.5 and T.sub.6 will be on and the transistor T.sub.2 will be on. Thereby, a large current will be instantaneously fed to the driving coil L.sub.2 from the capacitor C.sub.2 and the driving coil L.sub.2 will be energized. By this energization of the driving coil L.sub.2, a locking lever not illustrated will be moved and a shutter blade closing motion will be started. In such case, the current from the capacitor C.sub.2 will be checked by the diode D.sub.2 and therefore will not flow into the driving coil L.sub.1.
According to the above mentioned conventional circuit, in case the capacitance of the capacitors C.sub.1 and C.sub.2 is C, the voltage of the current source battery E.sub.0 is Vcc, the charge current is I and the voltage between the terminals of the capacitors C.sub.1 and C.sub.2 is Vc, the time t until charging the capacitors C.sub.1 and C.sub.2 is completed will be represented by the formula ##EQU1## and the charge current I will be represented by the formula ##EQU2## As evident from the above mentioned formulas (1) and (2) and the characteristic curve (a) in FIG. 3, in the case of such CR charging system as in the conventional circuit, a comparatively long time will be required from the starting of charging the capacitors C.sub.1 and C.sub.2 until the voltage Vc between their terminals reaches a predetermined valve. Therefore, in the case of using this circuit, the shutter chance will be likely to be missed. In order to shorten this charging time, the capacity of the current source battery may be enlarged so that the maximum output current (charge current) may be increased (see the characteristic curve (c) in FIG. 3). However, in the case by such process, a disadvantage that the current source battery will have to be large will occur.